Process for removing hexavalent chromium from chlorate containing liquors

ABSTRACT

A method has been devised for removing hexavalent chromium values from concentrated aqueous alkali metal chlorate solutions by treating said solutions with a soluble sulfide, neutralizing the treated mixture and thereafter separating an insoluble trivalent chromium product from the solution. Hypochlorites, when present, may be simultaneously removed from the chlorate solution by reaction with excess sulfide reagent.

United States Patent [191 Partridge et al.

[ PROCESS FOR REMOVING HEXAVALENT CHROMIUM FROM CHLORATE CONTAININGLIQUORS [75] lnventors: Harold deVere Partridge, Wilson;

Joseph M. I-Iildyard, Youngstown,

both of N.Y.

[73] Assignee: Hooker Chemical Corporation,

Niagra Falls, NY.

[22] Filed: Oct. 12, 1973 21 Appl. No.: 405,963

Related U.S. Application Data [63] Continuation-in-part of Ser. No.219,375, Jan. 20,

l972, abandoned.

[52] U.S. Cl 423/475, 423/472, 423/607 [51] Int. Cl. ..C01b 11/00, C01b11/14 [58] Field of Search 423/475, 472, 607, 184,

[ Oct. 22, 1974 [56] References Cited UNITED STATES PATENTS 2,784,0573/l957 Chisnall 423/179 3,002,815 l0/l96l Heinze 423/607 3,6l6,344l0/l97l Peterson et al. 423/607 X Primary Examiner-Edward SternAttorney, Agent, or F irm-Peter F. Casella, Richard P. Mueller [57]ABSTRACT 11 Claims, No Drawings I PROCESS FOR REMOVING HEXAVALENTCHROMIUM FROM CHLORATE CONTAINING LIQUORS BACKGROUND OF INVENTION Thepresent invention relates to a method for removing hexavalent chromiumvalues from concentrated aqueous solutions of alkali metal chlorates.More particularly it relates to the simultaneous removal of alkali metaldichromates and alkali metal hypochlorites from concentrated aqueoussolutions of alkali metal chlorates by treating said solutions withwater soluble sulfides.

It is known to prepare alkali metal chlorates, specifically sodiumchlorate, by the electrolysis of brine wherein the alkali metalhydroxide formed at the cathode reacts with the chlorine formed at theanode. The cathode which is generally fabricated from iron or similarcorrodible metal may, it is known, be protected from oxidation by theinclusion in the electrolyte of substances, such as an alkali metaldichromate, specifically sodium dichromate. The dichromates being highlycolored salts are objectionable impurities when permitted to remain inthe chlorate product and it is generally the practice to treat thechlorate solution with a soluble metal salt, such as a soluble bariumsalt, to remove the dichromate impurity as an insoluble barium salt.Such prior art processes have serious objections. Soluble barium saltsare not only relatively expensive but also combine with any sulfatesalts present producing difficultly filterable precipitates. Inaddition, excess barium salts must be precipitated, e.g., with sodiumcarbonate, and removed by filtration before the cell liquor can beprocessed further. Moreover, barium salts present pollution problems inthe disposal of wastes from processes utilizing alkali metal chlorates.

OBJECTS OF THE INVENTION Itis thus a principal object of this inventionto devise an economical and efficient method for removing hexavalentchromium values from concentrated aqueous solutions of alkali metalchlorates.

Another object is to devise a process for the simultaneous removal ofhexavalent chromium and alkali metal hypochlorite from alkali metalchlorate solutions.

Other objects will be obvious from the following description of thepresent invention.

SUMMARY OF THE INVENTION In accordance with the present invention, ithas now been found that hexavalent chromium values can be substantiallycompletely removed from concentrated aqueous solutions of alkali metalchlorates by treating said solutions contaminated with a hexavalentchromium compound with an amount of a soluble sulfide sufficient toreduce the hexavalent chromium to the trivalent stage, neutralizing thealkaline solution to a pH of below about 5, and thereafter separatingthe resultant insoluble trivalent chromium product from the chloratesolution. It has been found also that any alkali metal hypochloritepresent in the aqueous alkali metal chlorate solution may be removedsimultaneously by the addition of a sufficient excess of the solublesulfide.

Although the exact course of the reaction is not known, it is probablethat the reaction proceeds according to the following equations, usingsodium dichromate and sodium sulfide as the reactants in acid solution:

NQ-gCTzO- 3 3 H2504 H2O 2 Cr(OH) 38 3 Na SO 2 NaOH Similarly it may bepostulated that the reaction between sodium hypochlorite and sodiumsulfide proceeds as follows:

4 NaClO Na S Na S0, 4 NaCl NaClO Na S H O S NaCl 2 NaOH or combiningequations (2) and (3) 5 NaClO 2 Na S H O Na SO 5 NaCl 2 NaOH S (4)Accordingly, although the precise stoichiometry of the reactionsoccurring in the pr oces s of the present invention is not known, it isbelieved that at least three mol proportions of the soluble sulfide,e.g., sodium sulfide, are necessary to convert one mol proportion of thehexavalent chromium compound, e.g., sodium dichromate, to the trivalentstage and further at least about 0.4 molar proportion of the solublesulfide is required to kill each mol proportion of alkali metalhypochlorite, e.g., sodium hypochlorite, when present in the aqueousalkali metal chlorate composition. Preferably, in carrying outtheprocess of this invention an ex cess, and especially at least a 100percent excess, of the amount of soluble sulfide stoichiometricallyrequired to react with the hexavalent chromium values present in thechlorate liquor is used. Thereby, on acidification all of the hexavalentchromium is reduced to an insoluble trivalent chromium product and anyhypochlorite present is removed or killed.

DETAILED DESCRIPTION OF THE INVENTION In accordance with a preferredmode of carrying out this process of this invention, a concentratedaqueous solution of sodium chlorate, such as is obtained by theelectrolysis of brine and which contains about 200 to 400 grams perliter (gpl) of sodium chlorate about 100 to 200 gpl of sodium chlorideand about 0.2 to about l.5 gpl of sodium dichromate (Na Cr O '2I-I O) isheated to and maintained at from about 40 to about C. To this alkalinesolution about 0.5 to about 0.7 gpl of sodium sulfide, equivalent tofrom about 10 to about 20 mols of sodium sulfide per mol of sodiumdichromate Na Cr O -2H O), is added. The resultant alkaline yellowsolution, the pH of which is above about 8 and may be as high as 11 ormore, is neutralized to a pH within the range of about 4 to about 5 bythe addition of sulfuric acid. The solution becomes colorless and aprecipitate, which is probably a hydrated chromic oxide, appears and isremoved by filtration, preferably after adjusting the pH of the slurryto above and rate solution. Some or all of the excess sulfide, over thatrequired to react with the hexavalent chromium and hypochlorite isremoved during the process when the mass is rendered acid.

present in the solution can be used. for example. Such large apparentexcesses of sulfide are preferred in order to not only reduce thechromium values but also to simultaneously remove any hypochlorite fromthe chloespecially to about pH of 7, with an alkaline agent such 5 Thetemperature of the reaction is not critical. Conas caustic soda. Theclarified solution contains less than Vehiehtty, the Process is carriedout at about 400 to about 2 ppm f chromium and is f f sodium about 80 C,and preferably at about 65 C. The latter pochgorim temperature is thatat which the concentrated alkaline The reaction between alkali metalchlorates and al- Chlorate o u is discharged from y electrolysis kalimetal sulfides proceeds very slowly if at all in alka- Cells and 1t ls meohyehteht o treat the sotuttoh at line media. Since concentratedaqueous solutions ofsothis potht to remove the ehtomtum Vatuesp di hlare lk li L h a H f at least tures above and below the preferred rangecan be used about 8, and especially in the presence of stronglyalkaalthough the reactions proceed at a slower rate below line alkalimetal sulfides it is obvious that the reduction about C, and above about800 the energy of the hexavalent chromium values proceeds the morepended in heating the solution is wasted. slowly, if at all, in thestrongly alkaline reaction media Following formation of the chromichydroxide preof this process. It is therefore, highly desirable torenoipitate When the solution is acidified to 3 P below der the reactionmixture acid to expedite the reaction. about e slurry may be filtered.It has been found This can be observed by the decolorization of themixthat at this point, the sludge iS not readily filterable. ture onaddition of acid, However, lk li t l h1 Addition of filter aids improvesthe filtration at this rates decompose with the formation of chlorinedioxide g but it is preferred to neutralize the acid ur y to in thepresence of acids and sodium chloride, especially 3 P of about 5 orhigher, with an alkaline agent such at pHs below 4, and at temperaturesabove ambient. as caustic soda or the like y this Procedure not yAccordingly it is preferred to adjust the of the reaciS thedecomposition Of sodium chlorate minimized but i mass to a H f' b 5 Onceh color f h l also the separation of the insoluble chromic hydroxide i hb di h d L aft completion f h sludge is facilitated. Separation of theinsoluble matereduction of the hexavalent chromium values and aprial canbe accomplished a known muhhehes, for I pearance of the hydrated chromicoxide precipitate. ample, y filtrattoh, eehtrlfugatloh and the Thisfinal adjustment is preferably carried out to a pH The fottowithgexamples will Illustrate the Process of of above 5 and especially to apH of about 7 prior to the Present thyehtioh- Parts h Percentages are yfiltration of the insoluble chromic hydroxide. weight and temperaturesare glyeh in degrees eehti' Any water soluble sulfide can be used in theprocess grade, unless otherwise pe of the resent invention. Thus forexam le, sodium sulfide, p tassium sulfide, sodium bisulfide potassiumbi- EXAMPLE I sulfide, or the free acid, hydrogen sulfide can be used. Aconcentrated aqueous solution of sodium chlorate Hydrogen sulfide whenadded to the alkaline chlorate containing 350 grams per liter (gpl") ofsodium chloliquor would be converted to sodium sulfide or bisulrate, 140gpl sodium chloride and 0.l95 gpl sodium difide. "White Liquor," as usedin Kraft pulp manufacchromate (Na Cr- O '2H O) was heated to andmainturing plants, and which contains approximately 70 gpl tained at 65.Sufficient of an alkaline aqueous solution NaOH and 30 gpl Na S can beused as the source of the 40 of sodium sulfide (White Liquor containingabout 70 soluble sulfide also. Alkali metal polysulfides, e.g. percentNaOH and 30 percent Na S) was added to in- Na S can also be used.troduce 0.54 gpl sodium sulfide into the solution. The Sodium sulfideand sodium bisulfide because of their resulting yellow colored solutionhad a pH of l 1. This general availability and low cost are generallypresolution was rendered acid, by the addition of sulfuric ferred. acidto a pH between 4.0 and 4.5. The solution was agi- The amount of solublesulfide used should be at least tated as it became colorless and aprecipitate appeared. that amount which will reduce the hexavalent chro-Thereafterthe acidic mixture was neutralized to a pH mium values to thetrivalent state. Based on the stoichiof about 7 and filtered. Thetrivalent chromium ion ometry postulated above, it is believed that atleast 3 content of the clarified solution was determined by molproportions of sulfide are required to reduce one atomic absorption. molproportion of hexavalent chromium to the trivalent This experiment wasrepeated using sufficient stage. Preferably an excess is used, andaccordingly White Liquor to introduce 0.63 gpl sodium sulfide about 6mols of sulfide and especially from about 10 to into the mass. 20 molproportions of sulfide per mol proportion of The data obtained in theseexperiments are set out in hexavalent chromium compound present is used.the following Table l.

TABLET Initial Conc. Na,S Mol Ratio Cr Na,Cr,O,.2H,O Added Na,S/Na,Cr,O.2H O in solution 0.l95 gpl 0.54 gpl 10.56 l.9 ppm 0.i95 0.63 gpl 12.320.3 ppm About 10 or l2 to about 13 mol proportions of sulfide EXAMPLE llper mol proportion of hexavalent chromium compound The procedure ofExample I was repeated using a concentrated sodium chlorate solutioncontaining 360 gpl NaClO 2l6 gpl NaCl and 1.4 gpl Na Cr O '2H O. Theresults obtained are set out in the following Table II.

To a synthetic chlorate cell liquor containing about 350 gpl NaClO toabout 150 gpl NaCl and 0.15 gpl ble sulfide per molar proportion ofhexavalent chromium compound present,

2. neutralizing the resultant solution to a pH of about Na Cr O '2H O,at about 65 degrees sufficient aqueous 4 F about thereby Preclpltatmg"Willem sodium bisulfide containing 0.5 gpl sodium bisulfide ase alllhsehlble product, and saying 71.5 percent in solid state was added tointro- Separatmg F lhsbhlble mvaieht Chromium P duce 0.357 gpl NaHS intothe solution. The resulting from Sald Solutionyeuow solution having a ofabout 9 was rendg ed 2- The process Of Claim 1 wherein Said metal acidby the addition of sulfuric acid to a pH of 4.2. The Chlorate is sodiumChlorate. mixture was then divided into three equal portions and 1 TheProcess of Claim 2 wherein the Said Solution th portions w neutralized 1H 5, or 6, or 7 with is treated with an amount of soluble sulfide whichis at sodium hydroxide and then filtered. The clarified soluleastsufficient to reduce the hexavalent chromium valtions were assayed byatomic absorption for trivalent ues to the trivalent stage and also toreact with any alchromium. Table III is a record of the resultsobtained. kali metal hypochlorite present.

TABLE Ill Initial Conc. NaHS Mol Ratio Cr* *in Na,Cr,0,.2H,O addedNaHS/Na,Cr,O,.2H,O Filtered Solution 0.15 gpl 0.357 gpl 12.67 0.1 at pH5 0.15 gpl 0.357 gpl l2.67 0.1 at pH 6 0.15 gpl 0.357 gpl [2.67 0.1 atpH 7 EXAMPLE IV 4. The process of claim 2 wherein the said solution Aconcentrated sodium chlorate solution containing treated at least i 6m0] proportions of about 350 gpl Sodium Chlorate, about 140 gpl Sodiumble sulfide per mol proportlon of hexavalent chromium chloride, 3.5 x10' moles (0.26075 gpl) sodium hypo- Impound W 2 501mm; Chlorhe and 2 X0- mole 0 2 gpl) sodium 5. The process of claim 4 wherein the saidsolution dichromate (Na Cr 0;'2H O) was treated with 7.0 X treated Wlthfrom about 10 to about P 10- moles 0392 gpl) Sodium bi lfid at about 5.t1ons of soluble sulfide per mol proportlon ot hexava- The pH of thesolution was adjusted to 4.7 by the addilent Chromium compound Presentin said solutiontion of aqueous sulfuric acid. This amount of sodium ThePmeeSS of Claim 5 wherein the Said Solution bisulfide represents about100 percent excess over the is treated with from about 10 to about 13 Pp stoichiometric equivalent amount of NaHS required to tiOhS of SodiumSulfide P m0] Proportion of hexava' reduce the hexavalent chromium totrivalent chrolent chromium compound present in said solution. mium. Thesolution was agitated at about 65 for about 7. The process of claim 2 inwhich the said solution 30 minutes and then filtered. The filtratecontained less is treated with sodium bisulfide. than 0.1 ppm ofhexavalent chromium. 8. The process of claim 5 in which the saidsolution This invention has been described in the above speciis treatedwith about 12 to about 13 mol proportions of fication and the best modesfor carrying it out illussodium bisulfide per mol proportion ofhexavalent trated in the above examples. As will be obvious to chromiumcompound present in said solution. those skilled in this art manyvariations in the details set The P s Of Claim 5 wherein the SaidSOIUUOH, t h i can b d i h departing f h after treatment with thesoluble sulfide, and prior to cope or i i f h invention separating theinsoluble trivalent chromium product is wh t i l i d i adjusted to a pHbetween above about 5 and about 7. l. A process for removing hexavalentchromium values from alkaline aqueous solutions of alkali metal TheProcess of Claim 1 wherein the Process is chlorates which Comprises theSteps f r1ed out at a temperature of from about 40 to about I. treatinga concentrated aqueous solution of an 'alkali meml Chlorate having a pHabove about 8 with ll The process of claim 10 wherem the process 1s atleast about 3 molar proportions of a water soluearned out at i i

1. A PROCESS FOR REMOVING HEXAVALENT CHROMIUM VALUES FROM ALKALINEAQUEOUS SOLUTIONS OF ALKALI METAL CHLORATES WHICH COMPRISES THE STEPSOF
 1. TREATING A CONCENTRATED AQUEOUS SOLUTION OF AN ALKALI METALCHLORATE HAVING A PH ABOVE 8 WITH AT LEAST ABOUT 3 MOLAR PROPORTIONS OFA WATER SOLUBLE SULFIDE PER MOLAR PROPORTION OF HEXAVALENT CHROMIUMCOMPOUND PRESENT,
 2. NEUTRALIZING THE RESULTANT SOLUTIOON TO A PH OFABOUT 4 TO ABOUT 5 THEREBY PRECIPITATING TRIVALENT CHROMIUM AS ANINSOLUBLE PRODUCT, AND
 2. neutralizing the resultant solution to a pH ofabout 4 to about 5 thereby precipitating trivalent chromium as aninsoluble product, and
 2. The process of claim 1 wherein said alkalimetal chlorate is sodium chlorate.
 3. The process of claim 2 wherein thesaid solution is treated with an amount of soluble sulfide which is atleast sufficient to reduce the hexavalent chromium values to thetrivalent stage and also to react with any alkali metal hypochloritepresent.
 3. separating the insoluble trivalent chromium product fromsaid solution.
 3. SEPARATING THE INSOLUBLE TRIVALENT CHROMIUM PRODUCTFROM SAID SOLUTION.
 4. The process of claim 2 wherein the said solutionis treated with at least about 6 mol proportions of soluble sulfide permol proportion of hexavalent chromium compound present in said solution.5. The process of claim 4 wherein the said solution is treated with fromabout 10 to about 20 mol proportions of soluble sulfide per molproportion of hexavalent chromium compound present in said solution. 6.The process of claim 5 wherein the said solution is treated with fromabout 10 to about 13 mol proportions of sodium sulfide per molproportion of hexavalent chromium compound present in said solution. 7.The process of claim 2 in which the said solution is treated with sodiumbisulfide.
 8. The process of claim 5 in which the said solution istreated with about 12 to about 13 mol proportions of sodium bisulfideper mol proportion of hexavalent chromium compound present in saidsolution.
 9. The process of claim 5 wherein the said solution, aftertreatment with the soluble sulfide, and prior to separating theinsoluble trivalent chromium product is adjusted to a pH between aboveabout 5 and about
 7. 10. The process of claim 1 wherein the process iscarried out at a temperature of from about 40* to about 80* C.
 11. Theprocess of claim 10 wherein the process is carried out at about 65* C.